Many electronic devices on the market today often use power converters to convert electric energy from one form to another (e.g., converting between alternating current and direct current), amplifying a voltage or current of an electrical signal, modifying a frequency of an electrical signal, or some combination of the above. Examples of power converters may include boost converters, buck converters, and audio amplifiers (including, but not limited to Class D and Class H amplifiers). Such power converters often employ a switched output stage, an example of which is shown in FIG. 1. In FIG. 1, switched output stage 100 comprises a pull-up device 102 (e.g., a switch, a p-type metal-oxide-semiconductor field effect transistor, etc.) coupled at its non-gate terminals between a supply voltage and an output node, and a pull-down device 104 (e.g., a switch, an n-type metal-oxide-semiconductor field effect transistor, etc.) coupled at its non-gate terminals between a ground voltage and the output node. Predriver circuitry 106 may receive an input voltage vIN (typically a pulse-width-modulated input voltage signal) and apply control logic and/or buffering to such input voltage to drive a pull-up device driving signal voltage vp to the gate terminal of pull-up device 102 and to drive a pull-down device driving signal voltage vN to the gate terminal of pull-down device 104, wherein vp and vN are each a function of vIN. Accordingly, switched output stage 100 generates an output voltage vOUT to its output node which is a function of vIN.
One drawback in using switched output stages in a power converter is the presence of ringing, electromagnetic interference, or other undesirable effects which may be caused by many factors, including parasitic impedances between various nodes of the switched output stage. Overshoot and subsequent ringing may occur as a result of parasitic capacitances and inductances in the circuit resonating at their characteristic frequency, which decays over time due to resistances present in the circuit. For example, as output voltage signal vOUT transitions from a ground voltage to a supply voltage, vOUT may first overshoot the supply voltage by a particular amount of voltage, and then oscillate about the supply voltage as the ringing decays. Overshoot and ringing may be undesirable as they may cause unneeded current to flow (e.g., thereby wasting energy and potentially causing undesirable heat), may delay arrival at a desired final state, and/or may cause communication of incorrect signals.
Traditional approaches to reduction of overshoot and ringing include increasing the rise and fall times of device driving signals (e.g., vP and vN). However, such approaches are not without disadvantages, as increasing rise and fall times places constraints on timing parameters (e.g., minimum duty cycle) associated with the switched output stage. FIG. 2 illustrates example voltage and current graphs associated with switched output stage 100 illustrated in FIG. 1 during a rising-edge transition of vOUT, as is known in the art. As shown in FIG. 2, pull-down device driving signal voltage vN may decrease from a high voltage (e.g., a supply voltage) to a plateau voltage during a time period t1, and then remain at such plateau voltage during a period of time t2, before falling to zero. Also as shown in FIG. 2, vOUT may transition from zero to a supply voltage during time t2. Those of skill in the art may recognize that a long time period t1 places constraints on timing parameters (e.g., minimum duty cycle) associated with switched output stage 100 and thus negatively affects timing efficiency and power efficiency while not significantly improving electromagnetic interference. Conversely, long time period t2 will likely show reduced electromagnetic interference, overshoot, and ringing than a shorter time period t2. Assuming a constant current iN flowing from the gate terminal of pull-down device 104 to predriver circuitry 106 during each of time periods t1 and t2, any increase in time period t2 results in an increase in time period t1, and vice versa.
Although the foregoing discussion is limited to the waveform for pull-down device driving signal voltage vN, analogous problems, disadvantages, and challenges may exist with respect to pull-up device driving signal voltage vP.